Bolts and Threaded Parts

Shear loads can be transferred between two structural elements by either a bearing-type connection or a slip-critical connection.

Slip-Critical Joint (Type SC)

Slip-critical joint is a type of bolted structural steel connection which relies on friction between the two connected elements rather than bolt shear or bolt bearing to join two structural elements.

In a slip-critical connection, loads are transferred from one element to another through friction forces developed between the faying surfaces of the connection. These friction forces are generated by the extreme tightness of the structural bolts holding the connection together.

Bearing Type Connection

A bearing type connection is one in which the bolts are in shear because there is not significant enough friction in the joint to prevent slip. There are two major classifications of bearing type connections;

1. Type N - Connections with threads in shear plane

2. Type X - Connections without threads in shear plane

510.3.6 Tension and Shear Strength of Bolts and Threaded Parts

The design tension or shear strength, øR_n, and the allowable tension or shear strength, R_n/ Ω of a snug-tightened or pretensioned high-strength bolt or threaded part shall be determined according to the limit states of tensile rupture and shear rupture as follows:

510.3.7 Combined Tension and Shear in Bearing-Type Connections

The available tensile strength of a bolt subjected to combined tension and shear shall be determined according to the limit states of tension and shear rupture as follows:

F_nt = nominal tensile stress from Table 510.3.2, MPa

F_nv = nominal shear stress from Table 510.3.2, MPa

f_v = the required shear stress, MPa

The available shear stress of the fastener shall equal or exceed the required shear strength per unit area, f_v.

User Note: Note that when the required stress, f, in either shear or tension, is less than or equal to 30% of the corresponding available stress, the effects of combined stress need not be investigated. Also note that Eqs. 510.3-3a and 510.3-3b can be rewritten so as to find a nominal shear stress, F'_nv , as a function of the required tensile stress, f_t .

where:

μ = mean slip coefficient for Class A or B surfaces, as applicable, or as established by tests

= 0.35 for Class A surfaces (unpainted clean mill scale steel surfaces or surfaces with Class A coatings on blast-cleaned steel and hot-dipped galvanized and roughened surfaces)

= 0.50 for Class B surfaces (unpainted blast-cleaned steel surfaces or surfaces with Class B coatings on blast-cleaned steel)

D_u = 1.13; a multiplier that reflects the ratio of the mean installed bolt pretension to the specified minimum bolt pretension. The use of other values may be approved by the engineer-of-record.

h_sc = hole factor determined as follows :

(a) For standard size holes; h_sc = 1.00

(b) For oversized and short-slotted holes;

h_sc = 0.85

(c) For long-slotted holes; h_sc = 0.70

N_s =number of slip planes

T_b = minimum fastener tension given in Table 510.3.1, kN

User Note: There are special cases where, with oversize holes and slots parallel to the load, the movement possible due to connection slip could cause a structural failure. Resistance and safety factors are provided for connections where slip is prevented until the required strength load is reached.

Design loads are used for either design method and all connections must be checked for strength as bearing-type connections.

510.3.10 Bearing Strength at Bolt Holes

The available bearing strength, øR_n and R_n/Ω, at bolt holes shall be determined for the limit state of bearing as follows:

ø = 0.75 (LRFD) Ω = 2. 00(ASD)

1. For a bolt in a connection with standard, oversized, and short-slotted holes, independent of the direction of loading, or a long-slotted hole with the slot parallel to the direction of the bearing force:

a. when deformation at the bolt hole at service load is a design consideration

R_n = 1.2L_c t F_u ≤ 2.4 d t F_u (510.3-6a)

b. when deformation at the bolt hole at service load is not a design consideration:

R_n = 1.5L_c t F_u ≤ 3.0 d t F_u (510.3-6b)

2. For a bolt in a connection with long-slotted holes with the slot perpendicular to the direction of force:

R_n = 1.0L_c t F_u ≤ 2.0 d t F_u (510.3-6c)

For connections made using bolts that pass completely through an unstiffened box member or HSS, see Section 510.7 and Eq. 510.7-1.

where:

d = nominal bolt diameter, mm

F_u = specified minimum tensile strength of the connected material, MPa

L_c = clear distance, in the direction of the force, between the edge of the hole and the edge of the adjacent hole or edge of the material , mm

t = thickness of connected material, mm

For connections, the bearing resistance shall be taken as the sum of the bearing resistances of the individual bolts.

Bearing strength shall be checked for both bearing-type and slip-critical connections. Th use of oversized holes and short- and long-slotted holes parallel to the line of force is restricted to slip-critical connections per Section 510.3.2.

510.3.11 Special Fasteners

The nominal strength of special fasteners other than the bolts presented in Table 510.3.2 shall be verified by tests.

510.3.12 Tension Fasteners

When bolts or other fasteners in tension are attached to an unstiffened box or HSS wall, the strength of the wall shall be determined by rational analysis.

Example

Given the following data:

Plate thickness, t_p = 16 mm

Bolt diameter, d_b = 22 mm

Hole diameter, d_h = 24 mm

s₁ = 40 mm s₂ = 75 mm

s₃ = 50 mm s₄ = 100 mm

F_y = 248 MPa F_u = 400 MPa

Assuming that the deformation at the bolt hole is a design consideration:

• Determine the design bearing strength at bolt holes.

A. 1158 kN

B. 1336 kN

C. 987 kN

D. 890 kN

• Determine the allowable bearing strength at bolt holes.

A. 890 kN

B. 1158 kN

C. 1336 kN

D. 987 kN

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Example

It is required to determine the nominal and allowable tensile and shearing strengths of a 25-mm-diameter A325 bolts with threads not excluded from from shear planes and the connection is bearing type.

• The design tensile strength of the bolt is:

A. 256.7 kN

B. 304.3 kN

C. 346.3 kN

D. 228.2 kN

• The allowable tensile strength of the bolt is:

A. 152.1 kN

B. 346.3 kN

C. 304.3 kN

D. 304.3

• The design shear strength of the bolt is:

A. 182.6 kN

B. 210.7 kN

C. 136.9 kN

D. 126.3 kN

• The allowable shear strength of the bolt is:

A. 210.7 kN

B. 91.3 kN

C. 126.3 kN

D. 182.6 kN

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Example:

A WT section is connected to the column flange with four 22-mmdiameter A325 bolt with threads in shear plane. Assume bearing type connection and that the plate is adequate to resist the load.

Given:

Dead load, P_D =90 kN

Live load, P_L =210 kN

Angle, θ = 50°

• Determine the factored shear force per bolt.

A. 125 kN

B. 340 kN

C. 235 kN

D. 85 kN

• Determine the design shear strength per bolt.

A. 126 kN

B. 87 kN

C. 159 kN

D. 106 kN

• Determine the factored tensile force per bolt.

A. 71.3 kN

B. 86.7 kN

C. 315 kN

D. 285 kN

• Determine the design tensile strength per bolt.

A. 156.7 kN

B. 110.8 kN

C. 95.4 kN

D. 214.4 kN

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